Overload protection for a weigh scale having a flexure beam

ABSTRACT

A weigh scale includes a flexure beam disposed between a platform and the base. Load transmitting devices operate to deflect the beam as a function of the load placed on the platform. Overload protection is provided for the beam to restrict the amount of beam deflection to maintain it within its elastic limits to thereby prevent yielding of the beam.

BACKGROUND OF THE INVENTION

This invention relates to weigh scales. More particularly, it involvestechniques for protecting load sensing elements in weigh scales.

In commonly assigned U.S. Pat. No. 3,938,603, there is disclosed a weighscale utilizing a floating flexure beam which senses the weight of aload placed on the scale. Briefly, the patented scale includes a loadreceiving pan or platform spaced from the base and a pair of pivotmembers. The beam is connected between the pivot members. Turningmoments are generated in the pivot members when a load is applied to thescale platform. The turning moments cause the beam to deflect as afunction of the load. Sensing means such as strain gauges are applied tocentral portions of the beam to provide an indication of the amount ofbeam deflection.

The proportional deflection of the beam with respect to the load is oneof the most critical aspects of the operation of the scale. If the beamis allowed to deflect to such a degree that it exceed its elasticlimits, the beam will stretch or distort such that inaccuracies wouldresult in subsequent measurements. This phenomena is known as yielding.Although the scales are rated according to their maximum capacity, theyare often subjected to excessive loads which could cause yielding andthus destroy the precise accuracy of the scale.

Preloaded springs on the rails supporting the platform have been used toprevent further rotation of the pivot members when excessive loads areapplied. The preloaded springs are located about the four corners of thescale and serve to bottom out on the base after their preload strengthshave been exceeded. While satisfactory protection has been provided bythis technique, it does not readily lend itself to implementation inlarge capacity scales. This technique also may not respond quicklyenough to protect against shock loads applied to the scale platform.Furthermore, a compromise in the preload strengths of the springs mustbe reached to provide sufficient protection against excessiveoff-centered as well as centered loads on the scale.

SUMMARY OF THE INVENTION

According to the teachings of this invention, overload protection forthe beam is provided by way of a rigid member bridging a central portionof the beam. Preferably, the member takes the form of a tube having aninner portion which will come into contact with the beam if deflectedbeyond a certain amount thereby preventing yielding of the beam. In oneembodiment, one end of the tube includes an inwardly directed flangeportion which is affixed to the beam. In another embodiment, the entiretube is free floating. Various adjusting means are provided foradjusting the spacing between the tube and the beam.

BRIEF DESCRIPTION OF THE DRAWINGS

The various advantages of the present invention will come to light uponreading the following specification and by reference to the drawings inwhich:

FIG. 1 is a cross sectional view of an unloaded beamed weigh scaleutilizing one embodiment of the present invention;

FIG. 2 is an enlarged partial cross sectional view of the embodiment ofFIG. 1 showing the beam in an excessively loaded condition;

FIG. 3 is a partial cross sectional view showing another embodiment ofthe present invention;

FIG. 4 is a partial cross sectional view showing an alternativeembodiment;

FIG. 5 is a partial cross sectional view showing a modification of theoverload protection device shown in FIG. 4 with the beam in an unloadedposition;

FIG. 6 is a partial cross sectional view showing another modification ofthe overload protection device of FIG. 4 with the beam in an unloadedcondition.

FIG. 7 is a sectional view along the lines 7--7 of FIG. 6;

FIG. 8 is a view similar to FIG. 7 showing another modification;

FIG. 9 is a cross sectional view of still another embodiment with thebeam in an unloaded condition; and

FIG. 10 is a cross sectional view like FIG. 9 with the beam in anexcessively loaded condition.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The illustrative weigh scale apparatus 10 in FIG. 1 is designed forgeneral purpose use and includes a flat pan or platform 12 for receivingthe objects to be weighed. Platform 12 is generally rectangular as is abase 14 which includes upright side walls with an inwardly projectingflange portions 16, 17. Base 14 is effectively isolated from loadsapplied to the platform 12. This is accomplished by way of a pair ofpivot members 18, 19 extending parallel to their adjacent side walls.Pivot members 18, 19 are connected to platform 12 by way of flexiblestraps 20, 22 and 21, 23, respectively. The outermost straps 20, 21 areconnected at their upper ends to base flange portions 16, 17 whereas thelower portions of the straps are connected to lower portions of pivotmembers 18, 19, respectively. Flexure straps 22, 23 are connected attheir upper ends to upper portions of pivot members, 18, 19 and theirlower ends are connected to platform 12 by way of longitudinal rails 24,25, respectively. A beam 26 is connected at its opposite ends to pivotmembers 18, 19. Upon application of a load to platform 12, pivot members18, 19 rotate in opposite directions and cause the beam 26 to deflectdownwardly in proportion to the weight of the load. Central portions ofthe beam may be provided with a web portion by way of flat surfaces 28,29 which carry strain gauges 30, 31. Strain gauges 30, 31 provide anelectrical signal which is a function of the amount of deflection of thebeam 26. A protective bellows or boot 32 encompassing the web portionserves to protect the sensitive components of the strain sensingapparatus. Thus far, the weigh scale 10 is substantially that describedin U.S. Pat. No. 3,938,603 which is hereby incorporated by reference.

In FIGS. 1 and 2, overload protection for beam 26 is provided by way ofa pair of bosses 34, 35 which project upwardly from the bottom surfaceof base 14. Preferably, bosses 34, 35 are cast integrally with base 14.Bosses 34, 35 are aligned with beam 26 and are spaced as close aspossible to the webbed cental portion thereof without interferring withboot 32. A four inch spacing between bosses 34, 35 has provedsatisfactory.

Bosses 34, 35 include centrally tapped bores 36, 37, respectively.Screws 38, 39 have threaded shafts which engage the boxes 36, 37 intheir respective bosses. Shims 40, 41 disposed between the head ofscrews 38, 39 and the upper surface of bosses 34, 35 may be convenientlyutilized to adjust the spacing between the head of the screws and thelower portions of beam 26. The spacing is preferably between about 0.01inch and 0.03 inch depending upon the capacity of the scale. The spacingis with reference to the beam 26 being in an unloaded condition such asshown in FIG. 1. The shims 40, 41 may have resilient properties such asthose possessed by rubber or Belville washers.

FIG. 2 shows the operation of the overload protection devices when thebeam has encountered an excessive load condition. The amount ofdeflection is exaggerated in the drawings to more particularlyillustrate the operation of the invention. In such an overload conditionthe beam 26 strikes the head of screws 38, 39 to restrict furtherdeflection of the beam 26. The initial spacing therebetween is chosensuch that the beam is allowed to deflect within its elastic limits overthe normally encountered range of loads but will strike the projectionsfrom base 14 when excessive loads are applied to platform 12. In suchmanner it is insured that the beam 26 is deflected only within itselastic limits and does not pass the yield point in which beam 26 mayexhibit distortion causing inaccuracies in subsequent weigh scalemeasurements.

After the beam 26 strikes the projections the beam 26 ceases to floatand further loads are transferred directly to the base 26.

FIG. 3 illustrates another embodiment in which the projections take theform of set screws 42, 43. Tapped openings 44, 45 extend completelythrough the lower surface of base 14 and engage the threaded shafts ofscrews 42, 43. Sockets 46, 47 in screws 42, 43 are accessible from theexterior of base 14. Appropriate wrenches may be utilized to engagesockets 46, 47 to rotate them until the upper ends of screws 42, 43 arespaced the appropriate distances from beam 26. Jam nuts 48, 49 operateto lock screws 42, 43 in place after adjustment. Washers 50, 51 may beused to aid in the locking process.

Another concept for providing overload protection for beam 26 is shownin FIGS. 4-8. In this embodiment a tube 60 bridges the central portionof beam 26 which contains the strain gauges. Tube 60 includes inwardlydirected flange 62 which is rigidly attached to one side of theflattened portion of beam 26, for example, by welding. The longitudinalwalls of the tube 60 are spaced a predetermined distance from beam 26when in an unloaded condition. If beam 26 is deflected beyond a certainamount, the free floating end 64 will engage the compression side 66 ofbeam 26. Tube 60 is preferrably made of a material that has a higherspring rate than beam 26. For example, if the beam 26 is made ofaluminum, tube 60 may be made of steel. As shown in FIG. 4 with the beam26 undergoing substantial deflection, tube 60 provides a generallyparalleling moment reaction path over the central portions of the beamundergoing the most deflection. Once end 64 has engaged beam 26 furtherdeflection of the beam is restricted. Again, the space between the innerwalls of tube 60 and beam 26 will depend upon the capacity of the beamand is chosen so that the yield point of the beam is not exceeded whenexcessive loads are encountered. A smaller boot 68 can be used whichextends from the outer surface of tube end 64 to beam 26, the flange 62completing the enclosure for the sensitive sensor elements.

Various methods of adjusting the spacing between tube 60 and beam 26 areshown in FIGS. 5-8 in which the beam is shown in an unloaded condition.In FIG. 5, a set screw 70 extends through a tapped opening 72 in thewall of tube 60 near end 64. The distance between the lower end of screw70 and beam 26 may be readily adjusted by rotating screw 70 and lockingit in place by a variety of means. In FIG. 6, the spacing adjustment isaccomplished by way of an annular cap 74 having a concentric groove 76which snaps onto one end 64 of tube 60. A lip portion 78 of cap 74projects between the inner surface of the walls of the tube 60 and beam26. A variety of caps having different lip portion thicknesses may bekept in stock and used to provide the desired spacing.

Alternatively, the groove 76 may be eccentric as shown in FIG. 8 toprovide lip portion 78' with varying peripheral thicknesses as noted atdimensions labled A and B in the FIGURE. In such case the cap isangularly positioned until the lip portion with the desired thickness isdisposed between the compression side of beam 26 and tube end 64.

Another embodiment of this concept is shown in FIGS. 9 and 10. Here, asimple tube 80 effectively floats over central portions of the beam 26when in an unloaded condition as illustrated in FIG. 9. The tube 80 isnot connected to beam 26 via flange 62 as in the previous embodiment butis held laterally in place by way of two boots 82 and 84. When beam 26is excessively loaded as shown in FIG. 10 the inner walls of the tubecome into contact with the beam 26 to provide rigid structureparalleling the flattened strain gauge carrying portion of the beam torestrict further deflection and prevent yielding thereof. The relativediameter of tube 80 with respect to beam 26 is again chosen according tothe specifics of the beam as should now be evident to one skilled in theart.

It will be appreciated that the reduced cross sectional area of thecentral portion of beam 26 will undergo the most deflection due to themoments provided by pivot members 18, 19 and therefore the overloadprotection techniques are particularly concerned with protecting thisportion of the beam since the remaining portions thereof aresubstantially more rugged. It should be noted that while a tubularstructure is envisioned as the most convenient way of providing thenecessary strength, and will restrict beam deflection in all directions,other configurations will become apparent upon study of thespecification, drawings and the following claims.

I claim:
 1. A weigh scale comprising at least one beam, a base, aplatform, load transmitting means for deflecting the beam as a functionof the load placed on the platform, sensor means on central portions ofthe beam for providing a signal relating to the amount of deflection,and rigid overload protection means carried by the beam and generallybridging the central portion of the beam adapted to engage the beam whendeflected beyond a certain amount to restrict further deflection of thebeam to prevent yielding thereof.
 2. The scale of claim 1 wherein saidoverload protection means has a higher spring rate than said beam. 3.The scale of claim 2 wherein said overload protection means is a tube.4. The scale of claim 2 wherein said overload protection meanscomprises:a tube encompassing said sensor means and having an inwardlyprojecting flange rigidly connected to said beam on one side of saidsensor means, operative to space the inner surface of the other tube enda predetermined distance from the beam in an unloaded condition, withsaid other tube end engaging the beam when deflected beyond a certainamount to prevent yielding thereof.
 5. The scale of claims 3 or 4 whichfurther comprises adjustment means for adjusting the spacing betweensaid tube and the beam.
 6. The scale of claim 5 wherein said adjustmentmeans comprises a screw extending through a tapped opening in the tube.7. The scale of claim 5 wherein said adjustment means comprises:aninsert having a groove for mating with a portion of the tube, and saidinsert having a lip portion disposed between the beam and the innersurface of the tube.
 8. The scale of claim 7 wherein said insertcomprises an annular member having an eccentric groove to provide saidlip portion with varying thicknesses.
 9. A weigh scale comprising:abase, a platform spaced from the base, first and second pivot membersdisposed between the base and the platform, a beam having a givendiameter connnected between the pivot members, load transmitting meansfor producing moments in the pivot members to deflect the beam as afunction of a load applied to the platform, a central portion of thebeam having a reduced cross sectional area, sensor means on said centralportion of the beam for creating a signal relating to the deflectionthereof, a tube having an inner diameter greater than said beam, saidtube bridging said central portion of and being stiffer than said beam,said tube having portions adapted to engage the beam when deflectedbeyond a certain amount to prevent yielding thereof.
 10. The scale ofclaim 9 which further includes means for restricting lateral movement ofsaid tube on the beam.